A patient has arterial blood gas showing pH 7.32, PaCO₂ 48 mmHg, and HCO₃⁻ 24 mEq/L. Which primary acid-base disorder is present?

Explanation

## Systematic Acid-Base Interpretation ### Step-by-Step Analysis **Step 1: Assess pH** - pH = 7.32 → **Acidemia** (normal: 7.35–7.45) **Step 2: Identify Primary Disorder** - PaCO₂ = 48 mmHg → **Elevated** → points to **Respiratory Acidosis** - HCO₃⁻ = 24 mEq/L → **Normal** (24 mEq/L is within the normal range of 22–26 mEq/L) **Step 3: Assess Compensation** | Type | Rule | Expected HCO₃⁻ | |------|------|----------------| | Acute Respiratory Acidosis | HCO₃⁻ ↑ 1 mEq/L per 10 mmHg ↑ PaCO₂ | 24 + (8/10 × 1) ≈ **24.8 mEq/L** | | Chronic Respiratory Acidosis | HCO₃⁻ ↑ 3–4 mEq/L per 10 mmHg ↑ PaCO₂ | 24 + (8/10 × 3.5) ≈ **26.8 mEq/L** | **Calculation for this case:** - PaCO₂ is 48 mmHg (8 mmHg above normal of 40) - For **acute** respiratory acidosis: Expected HCO₃⁻ ≈ 24.8 mEq/L → observed HCO₃⁻ of 24 mEq/L is essentially at the expected value, consistent with **appropriate acute metabolic compensation** **Key Point:** The HCO₃⁻ of 24 mEq/L is entirely consistent with the expected metabolic response to an acute respiratory acidosis (PaCO₂ = 48 mmHg). This is therefore **respiratory acidosis with appropriate metabolic compensation** — Option A. There is no evidence of a concurrent metabolic acidosis; the HCO₃⁻ is not inappropriately low. **High-Yield:** The primary disorder is **respiratory acidosis** (elevated PaCO₂ driving the acidemia). The bicarbonate is normal and matches the predicted compensation for acute respiratory acidosis. Labeling this as "inappropriate" compensation is incorrect because the HCO₃⁻ falls within the expected range. **Clinical Pearl:** Per Harrison's Principles of Internal Medicine, in acute respiratory acidosis, for every 10 mmHg rise in PaCO₂, HCO₃⁻ rises by ~1 mEq/L (buffering). In chronic respiratory acidosis, the renal response raises HCO₃⁻ by 3–4 mEq/L per 10 mmHg rise. Here, the modest PaCO₂ elevation of 8 mmHg with HCO₃⁻ of 24 mEq/L is consistent with appropriate acute compensation, not a mixed disorder.